President and Mrs. Reagan were welcomed to the silver anniversary celebration of the Joffrey Ballet last night with some loud, scattered boos as they entered the presidential box at the Kennedy Center's Opera House. The boos were repeated, somewhat more loudly, when the Reagans waved to the audience during intermission.

When the Reagans left the box for an intermission cocktail party, reporters asked them about the incident.

"It was just one young boy, just one young boy," Nancy Reagan called out several times, holding up one finger for emphasis.

"He probably had a broken spring in his seat," laughed the president as he and the first lady hurried off to their reception.

It was the first time since he took office that Reagan was booed at a public event, with the exception of organized demonstrations.

After the Opera House curtain rose, hundreds in emeralds and silk were still cramming single file through metal detectors. Security for the full house was particularly tight because of the Reagans' attendance.

At least one third of the audience--a dazzling mix of political, social and artistic Washington, most of whom paid $100 for their orchestra seats--ended up standing in the back of the theater through the first sequence.

Last night's program was dedicated to the people of Afghanistan, "who are fighting for their freedom," according to the Stagebill. Robert Joffrey, founder and artistic director of the company, is of Afghan heritage.

According to one of the gala's organizers, the ballet company had invited Soviet Ambassador Anatoliy Dobrynin, who had originally accepted. Dobrynin was later told the program was dedicated to the Afghan freedom fighters, and he did not attend last night's performance.

Earlier in the day, Reagan signed a bill proclaiming March 21 Afghanistan Day, to condemn the Soviet invasion of that country, and also dedicated the next flight of the space shuttle to the Afghanistan resistance.

"I support the efforts of all those who are fighting for freedom there and the one thing an artist can do is dedicate a performance to a cause," said Joffrey backstage after the show, where the Reagans went to congratulate the cast.

"Nobody wants to embarrass anyone," said Joffrey.

Among the four ballets performed was "The Green Table," an antiwar statement picturing negotiating diplomats, refugees, guerrillas. Joffrey said he wasn't trying to make a statement with the inclusion of that ballet, but merely show the "range" of the company.

On the domestic front, senators at the gala were worrying about the current Senate floor battle. The full Senate is expected to vote today on the expulsion of Sen. Harrison Williams (D-N.J.) for his involvement in Abscam.

"It's all I've been thinking about," said Sen. Howard Metzenbaum (D-Ohio). "It was absolutely abominable behavior by the FBI and the government, but that's separate from the issues surrounding Pete Williams."

"It's a tragedy for the Senate, and I hope he resigns before tomorrow," said Sen. Ted Williams (R-Alaska). "I really am saddened that he didn't have the strength to get himself out of this predicament. But we have introduced a resolution to look into the practices of the FBI and how they're able to do things like this without being monitored."

The crowd started their evening at a pre-theater buffet dinner in the Atrium of the Kennedy Center and finished four hours later with a post-performance reception in the same place. The entire evening, as well as the Joffrey's 25th anniversary national tour, was underwritten by Philip Morris Inc.

Everyone hugged and kissed like old friends at a wedding. Reagan red and basic black were out. Bright silks and beige taffeta prevailed. Among the guests were Kitty Carlisle Hart, Evangeline Bruce, chief of protocol designee Selwa (Lucky) Roosevelt, Frank Hodsoll, chairman of the National Endowment for the Arts, deputy white House chief of staff Michael Deaver, Secretary of Defense Caspar Weinberger and White House social secretary Muffie Brandon.